Linking IPDs with Workflows

You can't explicitly link IPDs together as there is no higher-level diagram in Composer that shows IPDs as blocks and then allows you to interconnect them.
To clarify:

IPDs are linked via the workflows they are connected to. For example, assume IPD1 references Workflow1 and this workflow uses an Queue Interaction block. The block can be set to route the interaction to a Queue2 and Queue2 could exist in IPD2. Therefore IPD1 gets connected to IPD2.

An interaction queue can be defined in only one IPD. Once defined and Published, a corresponding object is created in Configuration Server. This interaction queue cannot exist in any other IPD. It can, however, be referenced by blocks in any number of workflows in the same Project.

Note: IRD allows moving a queue from one business process to another. Once moved, the queue can be used in the new business process. To do the same in Composer, you can copy an Interaction Queue block from one IPD and paste into another. Then delete the block from the original IPD. The same Interaction Queue block cannot exist in two IPDs.